1. Field of the Invention
The present invention relates generally to calibration techniques and apparatus for transformers and, in particular, to a method and apparatus for calibrating a linear variable differential transformer.
2. Description of Related Art
A linear variable differential transformer (LVDT) is a mutual inductance device (transformer) that produces an electrical output proportional to the displacement of a moveable core. Like other transformers, an LVDT consists of a primary coil, secondary coils (typically two), and a magnetic core or armature. An alternating current carrier excitation is applied to the primary winding or coil. It is this current in the primary coil that produces a varying magnetic field around the armature. The two secondary windings or coils are symmetrically spaced from the primary coil and are connected in a series opposing circuit. The magnetic field produced by the primary coil induces an alternating voltage in the secondary coils, which are also adjacent the moveable armature. The voltage of the induced signal from the secondary windings is related to the number of coils.
Motion of a non-contacting core or armature varies the mutual inductance of each secondary coil to the primary coil, which determines the voltage induced in each of the secondary coils. This voltage differential is linearly proportional to the displacement of the core and, therefore, the amplitude of the induced signal varies linearly with the displacement of the armature.
The two secondary coils have outputs which are balanced against each other and indicate the direction of the displacement of the core. Since the secondary windings are wound oppositely, an identical varying magnetic field applied to both windings creates an output voltage with the same amplitude but different sign (+/−). When the outputs from these secondary windings are added together, an equilibrium position and zero output signal are produced. This equilibrium position is typically when the armature or core is centered and undisplaced.
As discussed above, the primary design of the LVDT is that of a transformer. Therefore, an LVDT is subject to the same electrical characteristics as a transformer. For example, variance in temperature produces varying resistance. As temperature increases, resistance also increases. An increase in resistance increases the primary impedance resulting in a change in the induced secondary voltage, or output voltage. This variance dictates that the LVDT must have an accurate calibration prior to the start of testing or usage. Any significant change in its test environment temperature will affect the validity of its output voltage.